The pulp and paper making industry has for many years made regular use of apparatus for thickening pulp and paper stock, usually for storage or other temporary purposes. The apparatus most commonly used in the prior art practice is known as a decker, and is relatively closely comparable in structure and mode of operation with a cylinder type paper machine, in that its main components are a wire-covered cylinder mold and a vat in which the cylinder mold rotates. In operation, the thickened pulp collects on the outer surface of the mold and is then dumped or couched therefrom for transport to a storage tank or other next station.
Deckers occupy considerable floor space, and are also relatively expensive, since the cylinder mold is a fairly complex piece of equipment, including as it does a structural framework for the filter wire. In addition, a decker is necessarily slow in operation, partly because the rotational speed of the cylinder mold must be kept below values at which centrifugal force would tend to cause the thickened pulp to be thrown off its surface, and the surface speed of cylinder molds is commonly in the range of only 200-250 feet per minute and has a practical maximum of 300 feet per minute. In general a decker is capable of thickening feed stock of 0.7 to 1.0% consistency to a range of 4 to 6%.
Among alternatives to deckers which have been offered to the industry in recent years, particularly as a pulp washer with thickening capabilities, is apparatus constructed in accordance with Biondetti et al, U.S. Pat. Nos. 4,501,040 and 4,686,005, wherein a continuous wire loop is supported by guide rolls in such manner that it wraps a portion of a solid cylinder mounted outside the wire loop. Dilute feed stock is delivered to the wedge zone defined by the portion of the wire approaching the cylinder and the adjacent surface of the cylinder, and this suspension is dewatered by expression of liquid through the wire into a receiver mounted inside the wire loop. According to those patents, feed stock at a consistency of less than 1.5%, preferably 0.4 to 0.8%, can be thickened to a consistency of 5 to 8%.
More recently, the assignee of this application has introduced to the industry a new apparatus for thickening pulp and paper stock which has the structural and operational characteristics disclosed in Seifert et al, U.S. Pat. No. 4,722,793. This apparatus comprises, as its major components, a pair of rolls rotatably mounted in spaced relation on parallel axes. An endless mesh-type wire belt is trained around these rolls in wrapping relation with a substantial portion of their surfaces so that the rolls and belt cooperate to define a space bounded by the rolls and the opposed runs of the wire between the rolls.
A headbox is mounted in this space and includes an outlet which delivers the pulp suspension to be thickened into the wedge zone defined by the portion of the wire approaching one roll and the adjacent portion of the roll surface, so that this pulp suspension is trapped between the wire and the roll. The rolls are driven at a sufficiently high speed to develop centrifugal force which causes liquid to be expressed from the stock layer between the wire and each roll, and a corresponding thickening of the pulp carried on the inner surface of the wire as it travels around each roll. Means are provided for collecting this thickened pulp from the second roll and removing it to one side of the apparatus.
The apparatus disclosed in the Seifert et al patent is capable of operating at very much higher speeds than conventional thickening apparatus of the decker type, namely speeds in the range of 1500-4000 feet per minute as compared with decker operation at a linear speed having a practical limit of 300 feet per minute. It is also capable of thickening feed stock from an input consistency of the order of 0.5% to more than 12%.
As a result of these capabilities, the capacity of such apparatus, in terms of tons per day of pulp, is correspondingly high. In addition, while the patent described both rolls as liquid-impervious, it also recommended that the first roll be provided with a grooved surface. It has been found in practice that this results in substantially increasing the capacity of the apparatus, by reason of the fact that with a grooved roll, substantially more pulp can be trapped between the wire and roll than when the first roll, to which the headbox delivers the stock, is smooth surfaced.